| Year | Value | | 2021 | 2.89 | | 2020 | 3 | | 2019 | 3.3 | | 2018 | 3.3 | | 2017 | 3 | | 2016 | 3.1 | | 2015 | 3.2 | | 2014 | 3.2 | | 2013 | 3.1 | | 2012 | 3.2 | | 2011 | 3.1 | | 2010 | 3.1 | | 2009 | 2.9 | | 2008 | 2.8 | | 2007 | 2.7 | | 2006 | 2.7 | | 2005 | 2.5 | | 2004 | 2.6 | | 2003 | 2.5 | | 2002 | 2.4 | | 2001 | 2.4 | | 2000 | 2.4 | | 1999 | 2.6 | | 1998 | 2.7 | | 1997 | 2.6 | | 1996 | 2.8 | | 1995 | 3.1 | | 1994 | 2.9 | | 1993 | 3.2 | | 1992 | 3.52 | | 1991 | 3.58 | | 1990 | 3.63 | | 1989 | 3.875 | | 1988 | 4.007 | | 1987 | 4.146 | | 1986 | 4.218 | | 1985 | 4.205 | | 1984 | 4.232 | | 1983 | 4.179 | | 1982 | 4.192 | | 1981 | 4.204 | | 1980 | 4.217 | | 1979 | 4.374 | | 1978 | 4.494 | | 1977 | 4.577 | | 1976 | 4.614 | | 1975 | 4.657 | | 1974 | 4.726 | | 1973 | 4.847 | | 1972 | 4.962 | | 1971 | 4.971 | | 1970 | 5.01 | | 1969 | 5.042 | | 1968 | 5.088 | | 1967 | 5.201 | | 1966 | 5.301 | | 1965 | 5.411 | | 1964 | 5.465 | | 1963 | 5.51 | | 1962 | 5.484 | | 1961 | 5.434 | | 1960 | 5.376 |
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